178 Mr. J. T. Bottomley. Permanent Temperature [June 19, 



or " surface-conductivity " of bodies of various dimensions, and under 

 various circumstances as to surroundings and as to air pressure. 



With respect to the determination of the permanent temperatures 

 of wires carrying electric currents, very little seems to have been 

 done experimentally. In fact, the only important experiments on the 

 Bubject seem to be those of Mr. Preece, on the currents required to 

 fuse and to raise to a dull red heat platinum wires of different 

 diameters. These were communicated to the Royal Society in a paper 

 road April 3, 1884. I may here remark that the results given by 

 Mr. Preece differ in a very definite way from those which I have 

 obtained ; but it is to be noticed that the temperatures of Mr. Preece's 

 determinations were much higher than those at which I have, up to 

 the present, experimented. 



The method which I have employed for determining the heating 

 effect of a given current in a given wire has been to find the resist- 

 ance of the wire, first with a very feeble current passing through it, 

 and afterwards with the given more powerful current passing ; and 

 after it has been passing for a sufficiently long time for the wire to 

 have acquired a temperature permanent under the given circumstances. 

 The resistance of the wire becomes increased as the temperature 

 rises, and from the increase of resistance the increase of temperature 

 can be calculated. 



Fro. 1. 



POTENTIAL GALVANOMETER 



AMPERE METER 



C&HiHt StU/R 



RESISTANCE COfi 



In doing this I have used two methods of experimenting. In the 

 first method the wire under experiment was connected in series with 

 the battery supplying the current, an ampere-meter, and a standard 

 resistance of thick German silver wire. The German silver wire was 

 immersed in heavy paraffin oil to keep it cool, and the wire under 

 experiment was left freely exposed to the air, but carefully guarded 

 by paper screens placed at safe distances, both from draughts aud 



